1. Technical Field
The present invention relates to a novel extruded propylene polymer resin foam. More particularly, the present invention is concerned with a novel extruded propylene polymer resin foam comprising a vast plurality of cells and a matrix constituted by cell walls of the cells and comprised of a propylene polymer resin, wherein said propylene polymer resin has high viscoelasticity characteristics, and wherein the foam has a thickness as large as at least 20 mm, a density as small as 0.03 g/cm.sup.3 or less, an average cell diameter of from 0.4 to 2.0 mm and a closed cell ratio as large as at least 80%. The resin foam of the present invention has not only light weight, but also exhibits high cushioning performance and excellent mechanical strengths, so that, after fabricated into various sizes by cutting, the resultant resin foam articles can be advantageously used in the fields of cushion packaging materials, floating materials, and heat insulating materials.
2. Background Art
Extruded polyethylene resin foams have heretofore been well known, however, have various problems in respect of the properties thereof. On the other hand, propylene polymer resins have high rigidity as compared to polyethylene resins and, therefore, have advantages in that when foams are produced from propylene polymer resins, the foams can exhibit high mechanical strengths with the use of the resin in a reduced amount, so that excellent foams having light weight and excellent mechanical properties can be produced. Accordingly, various attempts have recently been made to improve propylene resin foams.
For example, Unexamined Japanese Patent Application Laid-Open specification No. H4-363227/1992 and Japanese Patent Application prior-to-examination Publication (kohyo) No. H5-506875/1993 (corresponding to International Patent Application Publication No. WO 91/13933) disclose extruded propylene polymer resin foams having a density of 0.03 g/cm.sup.3 or less. However, any of the conventional extruded propylene polymer resin forms having a density of 0.03 g/cm.sup.3 or less does not have a thickness and a closed cell ratio which are sufficient to exert high cushioning performance, so that these forms are unsatisfactory for use as cushion packaging materials.
With a view toward solving the problems accompanying the conventional propylene polymer resin forms, the present inventors have conducted experiments using extruded resin foams having different thicknesses which were produced from olefin polymer resins, such as ethylene polymer resins and propylene polymer resins, in order to find relationships between the thickness of the resin foam and the cushioning performance exerted by the resin foam. Observations have been made using a graph of dynamic impact characteristic curves showing the results of the experiments.
Referring to FIG. 3 of the accompanying drawings, there is shown a graph of dynamic impact characteristic curves, showing the relationships between the static stress generated on the resin foam and the peak acceleration sustained by the weight in the one-time dropping test, with respect to resin foams having different thicknesses. The dynamic impact characteristic curve was obtained by the experiments which were conducted in accordance with the "Testing Methods of Dynamic Compression for Package Cushioning Materials" prescribed in Japanese Industrial Standards (JIS) Z 0235, using extruded polyethylene resin foams each having a density of 0.025 g/cm.sup.3 and respectively having thicknesses of 20, 30 and 40 mm. From FIG. 3 and an explanation thereon (which is given below), not only will the meaning of the "cushioning performance" be made clear, but also it will be understood that for exerting an satisfactory cushioning performance it is necessary for a resin foam to have a thickness of at least 20 mm.
In the graph of FIG. 3 which shows the dynamic impact characteristic curves, the ordinate indicates the peak acceleration J (0-100 G), and the abscissa having a logarithmic scale indicates the static stress I (0.02-0.4 kgf/cm.sup.2). This graph has been prepared as follows. A predetermined number of different weights, each having an accelerometer equipped inside thereof, are individually dropped on resin foams to measure maximum accelerations with respect to the respective weights. Such maximum accelerations are defined as "peak accelerations". On the other hand, the respective weights are statically placed on the resin foams, and static stresses generated by the weights are obtained. The obtained peak acceleration values are plotted against the static stress values to obtain dynamic impact characteristic curves.
The experiments for obtaining FIG. 3 were conducted using different weights respectively capable of generating different static stresses in the range of from 0.02 to 0.4 kgf/cm.sup.2 on the foam. The range of from 0.02 to 0.4 kgf/cm.sup.2 covers the magnitudes of static stress which are generated when household electric apparatus, personal computers, OA (office automated) equipment, precision machines, etc. having a weight of about 5 to 50 kg are statically placed on resin foams. In FIG. 3, characters "t20", "t30" and "t40" respectively mean curves obtained with respect to foams having thicknesses of 20 mm, 30 mm and 40 mm. The minimum value of the peak accelerations which can be obtained from the dynamic impact characteristic curve indicates the minimum peak acceleration (load) which is experienced by the foam (corresponding to the characteristic curve) when the foam is used as a cushion packaging material for articles, such as household electric apparatus etc. This minimum peak acceleration indicates the maximum cushioning performance which the foam can exhibit.
It has also been found that the peak acceleration which does not cause mechanical troubles or damage on the articles, such as household electric apparatus etc., is generally 80 G or less. FIG. 3 clearly shows that when a foam having a thickness of less than 20 mm is used for packaging an article generating a static stress in the above-mentioned range (0.02 to 0.4 kgf/cm.sup.2), the peak acceleration which is experienced by the packaged article in the dropping thereof exceeds 80 G as an acceptable level, so that the article is likely to suffer mechanical troubles or damages. Accordingly, it will be understood that foams having a thickness of less than 20 mm are not suitable for use as safety packaging material for articles, such as household electric apparatus.
Further, as a result of the investigations by the present inventors, it has been found that for maintaining the cushioning performance even when repeatedly sustain impact, it is necessary for foams to have a closed cell ratio of at least 80%. When the closed cell ratio of the foam is less than 80%, the ratio of the minimum peak acceleration sustained by the foam at 2 to 5 times repeated dropping tests to that at a one-time dropping test disadvantageously becomes large and, therefore, such a foam cannot be used as a cushion packaging material without a danger of causing mechanical troubles and damages of a packaged article.
Furthermore, as a result of the investigations by the present inventors, it has also been found that with respect to at least 20 mm-thick foams to be used as cushion packaging materials, it is necessary for the foams to have an average cell diameter of from 0.4 to 2.0 mm. When the average cell diameter is less than 0.4 mm, the ratio of the compression stress generated in an extrusion direction in the production of a foam by extrusion to the compression stress generated in a thicknesswise direction becomes too large, so that the produced foam is caused to have a cushioning performance varied depending on the direction. Therefore, in actual use of the foam, it is necessary to carefully choose a direction in which the foam structure is used, which is cumbersome, so that the commercial value of such a direction-dependent foam as a cushion packaging material becomes very low. On the other hand, when the average cell diameter of the foam is more than 2.0 mm, such a foam has disadvantages in that not only is the surface appearance poor, but also the touch of the foam is unpleasant due to the large thickness of the cell wall, so that the commercial value of the foam becomes low also.
On the other hand, in the above-mentioned prior art publications, namely, Unexamined Japanese Patent Application Laid-Open Specification No. H4-363227 (hereinafter frequently referred to simply as "Japanese H4-363227A") and WO 91/13933 publication, it is described that it is difficult to produce an extruded foam from a propylene polymer resin as compared to the production of the extruded foam from a low density polyethylene resin. For example, in Japanese H4-363227A, it is described that since a propylene polymer resin has a high crystallinity as compared to a low density polyethylene resin, the viscoelasticity properties of the propylene polymer resin are likely to change according to a change (even slight) of temperature, so that the range of optimum temperature for extrusion becomes very narrow, leading to difficulties such that good quality foams cannot be obtained because it is not actually easy to adjust the temperature of the resin to a temperature within the above-mentioned narrow optimum range. For solving these problems of the above-mentioned two prior art publications, the following methods for producing propylene polymer resin foams have been proposed. In Japanese H4-363227A, it is described that when a propylene polymer resin exhibiting a melt tension of at least 7 gf at 230.degree. C. is used, extruded plank foams having a density of from 0.18 to 0.018 g/cm.sup.3 and a thickness of from 10 to 100 mm can be obtained. In WO 91/13933, it is described that when a specific propylene polymer resin comprising a major moiety of a linear propylene polymer and a minor moiety of side chains highly branched from the linear propylene polymer is used, extruded sheet foams having a thickness of from 0.5 to 5.0 mm can be obtained.
By using the resin disclosed in WO 91/13933, a good quality foam can be obtained as long as the foam has a density of from 0.04 to 0.4 g/cm.sup.3 and a thickness of 5 mm or less. However, the technique of WO 91/13933 has a serious problem in that when it is intended to produce a plank foam having a thickness as large as 20 mm or more, the .breakage of the cell walls (membranes) markedly occurs, so that the closed cell ratio is drastically lowered. When a cell nucleating agent or the like is added to the foaming resin composition for the purpose of preventing the closed cell ratio from lowering, the size of the closed cells is caused to become very small and have a diameter of less than 0.4 mm, so that not only cannot a foam having an increased thickness be obtained, but also various serious problems occur such that anisotropy is observed in the compression stress and cushioning performance of the foam produced, that scalelike concave-convex portions occur in the surface of the produced foam, and that the produced plank foam is wholly deformed into a wavy shape (hereinafter frequently referred to as "corrugation phenomenon"). Thus, good quality foams cannot be obtained by the technique of WO 91/13933.
On the other hand, when the resin disclosed in Japanese H4-363227A is used, substantially the same unfavorable phenomena as in WO 91/13933 occur as major problems. This means that the described resin properties of a "melt tension of 7 gf at 230.degree. C." cannot solve the essential problems accompanying the conventional resins as in WO 91/13933. More specifically stated, although Japanese H4-363227A contains a description that an extruded plank foam having a density of from 0.018 to 0.18 g/cm.sup.3 and a thickness of from 10 to 100 mm can be obtained, actual experiments show that a foam having a thickness increased to about 30 to 100 mm is obtained only when the density of the foam is on the order of from 0.10 to 0.18 g/cm.sup.3, and that, however, when it is intended to obtain a highly expanded foam having a density reduced to a level as low as 0.03 g/cm.sup.3 or less, the foam produced necessarily becomes a plank or sheet foam having a small thickness, i.e., a thickness of only 10 mm or less, so that the produced foam cannot exert a satisfactory cushioning performance in use as a cushion packaging material. In fact, in all of working examples in Japanese H4-363227A, which appear to contain a relatively sufficient disclosure for replication, there are only described foams having a thickness as small as 2 mm or less, which is believed to be ascribed to the difficult technical background as mentioned above.
As mentioned above, with the use of the specific resins used in Japanese H4-363227A and WO 91/13933, highly expanded foams having a high ratio of closed cells and satisfying requirements such that the density be from 0.005 to 0.03 g/cm.sup.3 and the thickness be at least 20 mm, cannot be obtained even if any production conditions are employed.
Therefore, if a cushion packaging material comprised of a propylene polymer resin foam having a thickness of 20 mm or more is desired, there has conventionally been no other measure than laminating a plurality of thin sheet foams having a thickness of from about 2 to 3 mm to each other by heating or by means of an adhesive to obtain a laminate foam structure. However, such a laminate foam is disadvantageous in that since a connection layer connecting the adjacent thin foams in the laminate foam structure is hard, a danger of impairing or damaging an article to be packaged is unavoidable when the article is contacted with the hard connection layer of the laminate foam structure, that since a large anisotropy occurs in compression stress and cushioning performance between an exposed face portion of the hard connection layer and other face portions of the laminate foam, the commercial value of the laminate foam as a cushion packaging material becomes very low, and that the production of such a laminate foam structure involves, in addition to the conventional extrusion-foaming step, an additional lamination step, which leads to a considerable increase in production cost.
In the above-mentioned situations, the present inventors have made extensive and intensive studies with a view toward developing propylene polymer resin foams exhibiting not only high cushioning performance and mechanical strength properties, but also light weight and excellent appearance. As a result, it has unexpectedly, surprisingly been found that when a specific propylene polymer resin exhibiting a biaxial extensional viscosity of at least 4.5.times.10.sup.6 poise at a biaxial extensional strain of 0.2, and a biaxial strain hardening rate of at least 0.30 (wherein the biaxial strain hardening rate is defined herein) is used as a resin to be supplied to an extruder (such a resin is hereinafter frequently referred to as "base resin"), and the base resin is subjected to extrusion foaming molding, an extruded propylene polymer resin foam having large thickness can be obtained. This extruded propylene polymer resin foam comprises a plurality of closed cells defined by cell walls which constitute a matrix of the foam, wherein the matrix comprising a propylene polymer resin exhibiting a biaxial extensional viscosity of at least 3.0.times.10.sup.6 poise at a biaxial extensional strain of 0.2, and a biaxial strain hardening rate of at least 0.25 (wherein the biaxial strain hardening rate is defined herein), and wherein the foam has specific foam properties which have not heretofore been realized, that is, the foam is a single layer foam having a thickness of at least 20 mm, a density of from 0.005 to 0.03 g/cm.sup.3, an average cell diameter of from 0.4 to 2.0 mm, and a closed cell ratio of at least 80%.
This extruded propylene polymer resin foam exhibits 80 G or less in terms of the minimum peak acceleration when tested in accordance with the "Testing Methods of Dynamic Compression for Package Cushioning Materials" prescribed in Japanese Industrial Standards (JIS) Z 0235, in which weights are dropped from a height of 60 cm on the foam. Thus, the extruded propylene polymer resin foam exhibits not only high cushioning performance and mechanical strength properties, but also has light weight and excellent appearance.
The present invention has been completed, based on the above novel findings.
Accordingly, a primary object of the present invention is to provide an extruded propylene polymer resin foam which exhibits not only high cushioning performance and mechanical strength properties, but also can maintain high cushioning performance even when repeatedly sustain impact, and which has light weight and excellent appearance.
The foregoing and other objects, features and advantages of the present invention will be apparent from the following detailed description and appended claims taken in connection with the accompanying drawings.